1. Field of the Invention
This invention is related to substrates for epilayer epitaxial growth in which the epilayers are lattice mismatched to the substrate and, in particular, to alternative substrates for fabrication of electronic and optoelectronic devices, such as semiconductor diode lasers, for example vertical-cavity surface-emitting lasers (VCSELs).
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
Lasers, such as semiconductor diode lasers, have a wide range of industrial and scientific uses. The use of semiconductor diode lasers as sources of optical energy is attractive for a number of reasons. For example, diode lasers have a relatively small volume and consume a small amount of power as compared to conventional laser devices. Further, as monolithic devices, they do not require a combination of a resonant cavity with external mirrors and other structures to generate a coherent output laser beam. One disadvantage of the semiconductor diode laser, however, is the relatively low power of the output beam, as compared to other types of laser devices.
Group III-V (xe2x80x9cIIIxe2x89xa7Vxe2x80x9d) semiconductor materials have been used to construct semiconductor lasers. Processing of III-V semiconductor devices includes vital steps for depositing III-V materials on a semiconductor substrate. For the deposition of a thick III-V layer, the lattice constant of the substrate material has to be very close to that of the deposited III-V layers (epi layers) with the same crystalline structure. Otherwise, crystalline defects, especially threading dislocations, will form during material deposition. When the defect density in the deposited material is high, it will significantly degrade device performance. These threading dislocation defects can create leakage paths for current, provide undesired carrier recombination centers and reduce device lifetime.
It is thus very difficult to grow high quality thin film materials on conventional prior art substrates with a large lattice mismatch. This lattice-matching requirement for compound semiconductor material deposition severely limits the possible choice of compound semiconductor material compositions and device material structure designs due to the limited choice of available substrates with the appropriate crystalline structures and lattice constants. Such substrates include Si, GaAs, InP, GaSb, InAs, and sapphire, inter alia.
For material systems for which there are no lattice-matched prior art substrates, however, some alternative approaches have been used. E.g., either a thick buffer layer is grown on the substrate, as proposed in U.S. Pat. No. 5,285,086, or a special technique, such as the lateral growth method proposed by Parillaud et al., Appl. Phys. Lett. vol. 68 (1996), p. 2654, is employed before the growth of the device structure layers. It is known that defects, in particular threading dislocations, induced by lattice mismatch can be reduced from 1011/cm2 to 105/cm2 by using the lateral growth method, for example. However, lattice-mismatched material growth techniques that result in defects, especially threading dislocation defects, often cause undesirable performance or characteristics of optoelectronic or electronic devices grown with such techniques.
It is desirable to epitaxially fabricate a variety of types of structures or devices, using a given epi material system, grown on a given substrate. Such epitaxially fabricated devices include electronic devices, such as transistors and integrated circuits, and optoelectronic devices, such as semiconductor lasers, light-emitting diodes, and photodetectors.
One such optoelectronic device in which there has recently been an increased interest is the vertical-cavity surface-emitting laser (VCSEL). The conventional VCSEL has several advantages, such as emitting light perpendicular to the surface of the die, and the possibility of fabrication of two dimensional arrays. VCSELs typically have a circular laser beam and a smaller divergence angle, and are therefore more attractive than edge-emitting lasers in some applications. Long infra-red spectrum wavelength (e.g., the range from approximately 1.2 xcexcm to approximately 1.8 xcexcm, including closely-spaced wavelengths around 1.3 xcexcm or closely-spaced ITU grid wavelengths around 1.55 xcexcm) VCSELs are also of great interest in the optical telecommunications industry because of the minimum fiber dispersion at 1.32 xcexcm and the minimum fiber loss at 1.55 xcexcm. The dispersion shifted fiber will have both minimum dispersion and minimum loss at 1.55 xcexcm. The long wavelength VCSEL is typically based on an InxGa1xe2x88x92xAsy P1xe2x88x92y active layer lattice matched to InP cladding layers.
The structure of a typical VCSEL usually consists of an active region sandwiched between two distributed Bragg reflector (DBR) mirrors, as shown schematically in FIG. 1. For the fabrication of long wavelength (e.g., 1.3 or 1.55 xcexcm) VCSELs, it is very difficult to form the desired materials in one single growth step on a substrate. For instance, it is difficult to grow either the desired 1.3 xcexcm active region on a GaAs substrate or to grow proper DBR mirrors on an InP substrate, despite the maturity of the technology for growing the DBR structure on GaAs substrates. Likewise, it is difficult to grow a 1.3 xcexcm wavelength DBR structure on an InP substrate, despite the maturity of the technology for growing the active region. Recently, some alternative material systems, such as InGaNAs, GaAsSb and InGaAs quantum dots, have been developed to grow directly on a GaAs substrate using an AlxGa1xe2x88x92xAs/AlyGa1xe2x88x92yAs DBR for a 1.3 xcexcm wavelength active region. However, these material systems are very difficult to grow and not easy to reproduce.
Another alternative approach to fabricate a long wavelength VCSEL is by using the so-called wafer bonding technique. However, this approach requires at least two to three wafer growth and one to two wafer-to-wafer bonding processes, which leads to very high fabrication cost and very low device yield. Therefore, a single wafer growth approach would be preferable to the wafer bonding approach, other considerations being equal.
One alternative approach to fabricate a long wavelength VCSEL with a single wafer growth step is to use the (In,Ga,Al)As material system lattice matched to Inx(AlyGa1xe2x88x92y)1xe2x88x92xAs, where, e.g., 0.15 less than x less than 0.45, and growth of an InAlGaAs/InAlAs DBR structure and a moderately strained InGaAs quantum well (QW) structure active region. (Depending on the value of x, y is selected such that the material utilized has a bandgap absorption edge less than the lasing wavelength, e.g. less than 1.3 xcexcm for a 1.3 xcexcm VCSEL.) However, there is no commercially available substrate that is lattice matched to this material system. It is very difficult to control the composition precisely of a ternary InxGa1xe2x88x92xAs substrate uniformly over a whole wafer. Therefore, a high quality alternative substrate needs to be developed for this application.
One approach is to create a substrate that has the same crystalline structure and the same surface lattice constant as those of non-strained Inx(AlyGa1xe2x88x92y)1xe2x88x92xAs, where 0.15 less than x less than 0.45. Another approach is to make a substrate that has a thin layer that is physically attached to the substrate, but can freely expand in a direction parallel to the substrate surface during material growth. This thin surface layer must have the same crystalline structure and a similar lattice constant as those of non-strained Inx(AlyGa1xe2x88x92y)1xe2x88x92xAs, where 0.15 less than x less than 0.45.
For lattice-mismatched epitaxial layers, it is widely accepted that there exists a critical thickness beyond which misfit dislocations are introduced causing the breakdown of coherence between the substrate and epitaxial layers. The relaxation mechanism for lattice-mismatched epilayers known as the Matthews-Blakeslee model, and other aspects of epitaxial, layer lattice mismatching problems are discussed in J. W. Matthews, S. Mader and T. B. Light, J. Appl. Phys. 41 (1970): 3800; J. W. Matthews and A. E. Blakeslee, xe2x80x9cDefects in Epitaxial Multilayers I,xe2x80x9d J. Cryst. Growth 27 (1974): 118-125; J. W. Matthews and A. E. Blakeslee, xe2x80x9cDefects in Epitaxial Multilayers II,xe2x80x9d J. Cryst. Growth 29 (1975): 273-280; J. W. Matthews and A. E. Blakeslee, xe2x80x9cDefects in Epitaxial Multilayers III,xe2x80x9d J. Cryst. Growth 32 (1976): 265-273; and J. W. Matthews, J. Vac. Sci. Technol. 12 (1975): 126.
U.S. Pat. No. 5,294,808 for xe2x80x9cPseudomorphic and Dislocation Free Heteroepitaxial Structuresxe2x80x9d proposes to use a thin substrate having a thickness on the order of the xe2x80x9ccriticalxe2x80x9d thickness, which is the thickness at which defects form when growing one lattice mismatched material on another. The critical thickness is only a few hundred angstroms, and it is difficult to sustain the mechanical and chemical processes required for epitaxial (epi) growth and device fabrication on a substrate having a thickness of only a few hundred angstroms. However, in practical situations, after the thin substrate is bonded to the supporting substrate, the bonding strength between the interface is so strong that this thin substrate can no longer freely change its lattice constant in the in-plane direction. Therefore, threading dislocations will still be generated due to the very limited strain accommodation in the thin substrate.
There is, therefore, a need for improved substrates and fabrication techniques that address the foregoing problems. In general, there is a need for alternative substrates that can be used for a variety of epi material systems without giving rise to conventional problems caused by lattice mismatch between the epi layers and the substrate. For example, there is a need for alternative substrates that address the problems associated with lattice mismatching between a substrate and the (In,Al,Ga)As material intended to be used for long (e.g., 1.3 or 1.55 xcexcm) wavelength VCSELs or other special material systems. Such alternative substrates could be advantageously used for other material systems and device structures as well, and in general for any material system for which other substrates cannot satisfy the lattice-matching requirement for device applications.